Lesson X.VL] PRESSURE OF AIR. 85 



inches : or equal to the weight of a column of 

 water of the same base and height, betw.een thirty- 

 two and thirty-five feet. The first part of this is 

 proved by observations on the Barometer, an in- 

 strument which measures the pressure of the air: 

 tor the limit of barometrical variation is between 

 twenty-eight and thirty-one inches. And the latter 

 part is proved by the sucking pump, which will 

 never raise water higher than thirty. five feet, and 

 sometimes not higher than thirty-two. This va- 

 riation in the weight of the atmosphere depends 

 in great measure on the different degrees of heat 

 in the air near the surface of the earth ; but, 

 perhaps it depends in a much greater degree, on 

 the commotions and changes in the atmosphere 

 from wind, vapours, and other causes. 



From the foregoing account of the weight of 

 the atmosphere, it will not be difficult to infer, 

 that, at a medium, there is a pressure equal to nearly 

 fifteen pounds avoirdupois upon every square inch : 

 this entirely, arises from its weight and fluidity. 

 By the combination of these two qualities, which 

 bind down all bodies on the earth with such great 

 force, many wonderful effects are produced: it is 

 this which prevents the arterial vessels of animals 

 and plants from being too much distended, by the 

 impetus of the circulating juices, or by the. elastic 

 foice of the air so copiously abounding in them :. it 

 is this also which hinders the fluids from transpir- 

 at. ing in. too great a degree through the pores of their 

 containing vessels, which would otherwise destroy 



the 



